This is awkward. Harry Kewell's soap-star wife is crying and it is my fault. It is March 2006 and we are at a fashion awards night in London and it feels as though the party has come to a record-scratching halt. There is no intent to make Sheree Murphy's mascara run: we have simply been chatting about her husband's stop-start-stop career.

Kewell has been injured for the worst part of three years at Liverpool. Murphy asks why the fans are beginning to turn on her husband. They do not realise what he is going through. She has to pick up the pieces at home. If only Liverpool fans knew the real story ... if only anybody did, other than Kewell, his wife and his best mate. Murphy's eyes well and a black tear streaks down her cheek.

Back then, it looked as if Australia's most gifted footballer was limping towards a disappointing end to an unfulfilled career when he should have been at his peak. Four years later, over lunch in Sydney, Kewell recalls just how dark those days were. "I came close to quitting football a couple of times," he reveals. "Only my wife, my manager and my best mate know how close."

Bernie Mandic, his manager, says: "I knew he was being serious." It was his job to talk Kewell round. "I marvel at Harry's resilience. His way of getting himself out [of a black mood] is by thinking of his wife and kids. He sees them as the reason that makes everything worth going through, no matter how bad it is. As long as they are fine, nothing else really matters to him – and I do mean nothing."

Did Kewell ever suffer from depression? "You get to the point where it's such a struggle, you wonder if it's all worth it", is all he will say.

But a friend of Kewell reveals that his mental health was affected as he began to wonder if he would ever recover. The lowest point apparently came after the World Cup in 2006, when Kewell was told by British doctors that they had fixed him physically and his problems were now more in mind than body. It was then that he considered walking away.

With another World Cup on the horizon, Kewell has left Liverpool, his family and most of his injury problems behind and begun a new life in Istanbul, playing for Galatasaray where, at 31, he is in the goalscoring form of his life. In five years at Liverpool, he played only 93 league games and scored 12 goals, seven of which came in his first four months. He averaged one goal a season after that glorious start. Now Kewell is a player reborn, scoring at a rate of almost a goal every other game. Granted, Turkish Super Lig defences are not as tough as those in the Premier League. But, at the time of writing, Kewell has scored 27 times in 64 appearances over the past two seasons.

In Sydney, before flying out to South Africa, Kewell looked fit and well. He has been in pain for years, has undergone 16 operations, and was injured recently. But he is confident of being fit to lead the Socceroos' attack when they start their World Cup campaign against Germany.

Kewell is loving life in Turkey and it shows. The joy is back. "I never doubted my football, it's just a lot of things happened that prohibited me from playing. People are saying now: 'You're at your career best.' I'm just enjoying my football again."

The club's staffing arrangements suit Kewell. Gala have one of the world's best managers in Frank Rijkaard, who conducts all his team talks in English. "When I heard that we got him as manager, I was thrilled to bits," Kewell says. "Even his second in charge, Johan Neeskens, I get on really well with from when he was assistant coach to [Australia's former manager] Guus Hiddink at the [2006] World Cup." Plus, there is a fellow Aussie at the club to keep him company. The national captain, Lucas Neill, joined in January from Everton. "We both enjoy our social lives together," Kewell says. "Now I've got a golfing partner over there."

Galatasaray's Ali Sami Yen has a reputation as being the most intimidating stadium in world football. While he was very occasionally, and hurtfully, booed by some Liverpool supporters towards the end of his time at Anfield, Kewell is a fans' favourite at Gala.

Kewell's previous experiences of Istanbul had been anything but positive. Between 1995 and 2003 he played for Leeds and when they faced Galatasaray in the Uefa Cup semi-final in 2000, two Leeds fans were fatally stabbed the night before the game. Ten years on, the four men found guilty of involvement in the murders are yet to spend any time in prison, much to the fury of Leeds fans.

Many Leeds supporters felt betrayed by Kewell's move. "When I decided to join Galatasaray, obviously there were a few Leeds fans who were disappointed. The reason why I came to Galatasaray was maybe to bring these clubs together, where we can forget what happened in the past and move on. Out of respect, I even chose to wear the No19 jersey. When I played for Leeds I wore 19 so I took the No19 again here to show that I'm trying to bind these two clubs together."

Kewell's other previous visit to Istanbul was for the 2005 Champions League final, when Liverpool spectacularly overturned a 3-0 deficit at half-time to beat Milan on penalties. It was a great night for Liverpool but not Kewell, who limped off in the first half. "On a personal level, that was a nightmare, a horrible night. You try to be happy for the team, but as soon as you go back to the hotel, that's it. There's nothing."

After nearly five years on the treatment table, Kewell never managed to recover from his injuries at Liverpool, and his time at the club finished unhappily. Most people thought he would never consistently rediscover top form.

Now, Kewell will be treated only by Australian doctors. The underlying cause of his injuries has been the subject of much speculation. At various times it has been rumoured he suffers from everything from septic arthritis to gout. What is now confirmed is that Kewell has autoimmune hepatitis, and he only found out when he went for a routine medical in order to get a home loan.

Doctors are not sure what causes the condition. Kewell is reluctant to speak about it, which has only served to pique interest. "If I went into full detail people would be amazed," he says. "In time people will know the truth. I will tell my side of the story properly when the time is right." That time is not now, evidently.

Autoimmune hepatitis is an extremely rare condition. The body's immune system attacks liver cells it should be protecting, and the consequences can be serious. The condition may have caused Kewell's immune system to break down, making him more susceptible to injuries and slower to recover. Kewell now manages his health successfully: he is careful about what he eats and drinks.

Homesickness is also taking its toll on Kewell. His wife and their three children – Taylor, nine, Ruby, seven, and Matilda, two – stayed in England when Kewell moved to Istanbul. They visit regularly, but most of the time Kewell lives alone in a sleek two-storey apartment. "Got my wife over here to decorate it, make it feel more homely," he says.

Kewell and Murphy have been together for almost 11 years, married for nine. They met in 2000 at the Majestyk nightclub in Leeds, the year when Kewell, then 21, was voted by his peers as the Professional Footballers' Association best young player. Murphy was a star on Emmerdale but put her career on hold in 2003 to raise the children. She has recently resumed acting, having landed a part in Hollyoaks.

"It's very hard," Kewell admits, when asked what it's been like to be away from his family for the past two years. "You wander around … Usually I'd be juggling three children, going to the school grounds, doing homework. Now I'm finding I've got about five or six hours where I just twiddle my thumbs. But I work a lot. I do two sessions a day. I've got all this energy. I try to keep busy."

Kewell's accent is a homeless hybrid of Aussie flecked with northern English and he has a habit of ending sentences with "and that". Although he is warm, friendly and engaging in person, there is something cold and businesslike about his attitude to his football career. "It's work. I ain't over there [in Turkey] to be friends with anyone, to socialise and that, I'm over there for work." By choice, Kewell has very few close friends. "It's a selfish game," he says. "You have to look after yourself."

He and Mandic have done that all right. Kewell was third on the Australian sporting rich list in 2009, behind the golfer Greg Norman and the NBA star Andrew Bogut, and reportedly earned almost £6m last year. Kewell's contract at Galatasaray expires this month. Fans have started a web campaign – staywithusharry.com – to keep him. But, because his latest injury has taken longer to heal than expected, the club have so far not offered him a firm new deal.

In the past, Kewell has been criticised in his homeland for picking and choosing when to make himself available for international duty. Kewell has 45 caps and 13 goals. By rights, he ought to have doubled those numbers.

But though he may have missed a few international friendlies over the years, in the big games Kewell has always been there and has always delivered. When Australia just missed qualifying for the 1998 World Cup in the two legs against Iran, Kewell scored home and away. When the Socceroos beat Uruguay in the play-off to qualify for the 2006 World Cup, he turned the game. He was clearly not fully fit four years ago, but he put his body on the line to produce a goalscoring performance against Croatia to earn the point that secured Australia's passage through to the final 16.

Australia were the dark horse team of the last World Cup. "It was the first time the world actually saw Australia play," Kewell says. "We took it by storm." The Socceroos were knocked out by Italy, the eventual tournament winners, thanks to what they felt was a dive that led to a cruelly decisive penalty in stoppage time. This year, the world will know what to expect. "It's going to be harder because teams will be more wary of us," Kewell predicts.

This time around, Kewell needs the Socceroos just as much as they need him. He is up for sale in what will probably be his last big chance to cash in, and there is no bigger shop window than the world's biggest sporting tournament.